Some sensor/digitizer problems we have had in the past.

Problem: The sensor was off for around 6 month due do power problems. 
The spectrum of BHZ has sometimes an pulse which occur the straight lines in the spectrum.
Sensortype: Trillium 120
Solution: After unplug/plug the connector for several times the signal was good again. 
Looks like it was only an connection problem. 
We have seen similar problems also at other stations